Microwave cooking is based on the absorption of microwave energy by polar substances in food and food packaging. The absorbed microwave energy can be re-emitted by polar substances within the food components in the form of heat energy. The water content of food is primarily responsible for absorbing microwave energy and re-emitting the energy as heat. Additionally, foods can be heated by contacting microwave energy with susceptor surfaces or heating devices within microwave cooking containers which absorb microwave energy and then transfer absorbed energy into the food as heat. Such heating modes can result in the creation of locally high temperatures in certain hotspots in foods, on portions of susceptor surfaces or in other heated surfaces that range from 90.degree. F. to 500.degree. F. and in some cases even higher.
A large variety of foods and food container materials are currently being used in microwave cooking. Both solid, semi-solid and liquid food substances, available for breakfast, brunch, lunch, dinner or snack time consumption have been adapted for microwave cooking. In order to provide the foods in a pleasing, easily-used package, a variety of materials have found a use in microwave cooking including a variety of inorganic materials, organic films and fibers. Additionally, structural thermoplastic and thermosetting materials can be used. Further, ceramics, paper, cardboard, metallized polyester, etc., are used. Susceptor materials, used as sources of heat for certain applications, most commonly comprise a film or sheet, such as Mylar, a polyethylene teraphthalate polyester, with finely divided aluminum particles imbedded in or deposited on, the film or sheet material. Such sheets are placed in the microwave cooking container directly adjacent to the food substance to introduce heat directly into the food.
Because of the variety of food substances and the variety of packaging materials used in cooking containers, a variety of volatile substances with large amounts of steam or water vapor can be introduced into the vapor space of the microwave oven or the vapor space within the cooking container. Such materials are created or driven from the food or container materials by the action of heat on the volatile material and other substances in the food. Examples of volatile substances that could arise during cooking include water, thermal decomposition products of food materials and packaging materials, volatile substances that are volatilized or steam distilled from the packaging or food substances and new reactive species created by reaction, in the vapor space, of reactive volatile materials driven from the food or package material.
The production of such volatile materials during microwave heating of food and packaging materials has created significant interest and concern in the identification and quantitative determination of the amounts of materials produced. Further, a great deal of interest has been focused on identifying the origin, the production mode, the transportation mode from the food or other material through the vapor space of the packaging material and oven interior, and final point of migration of volatile substances. The volatilization and migration of such volatile substances can change the color, flavor and texture of foods, can change the nature or integrity of packaging materials, and in certain cases could potentially result in the concentration of undesirable or even toxic substances in foods or packaging materials after heating.
Such a need has been present during the years microwave ovens have been available. A technique known as microwave head space analysis a packaging material has been used by NFPA/SPI. This method involves placing 1 to 5 grams of materials to be analyzed within a small head space container in a microwave oven adjacent to a large volume (100-500 ml) of water. The microwave oven is operated and decomposition or volatilization of compounds occurs within the head space vial. The head space vial is then transferred to a head space analyzer which injects the material into a gas chromatograph analyzer wherein the materials are transferred to the analytical column and analyzed. We have found that for the purposes of the precise determination of origin, identity, chemical changes, migration and final locus measurements that this head space analytical procedure is insufficient in producing reliable data for many food and packaging systems. However, even view of the fact that a variety of analytical techniques for the identification and quantitization of variety of chemical substances have been known, the prior art has never disclosed a quick, simple method and apparatus for directly determining the identity of volatile substances, or for quantifying the amount of such substances and tracing the production mode and final locus of volatile material in either food packaging or microwave oven after cooking.
Accordingly, a substantial need exists for an analytical technique that permits identifying the chemical nature of and amount of a volatile substance optionally in the presence of large amounts of steam or water vapor produced during microwave heating derived from foods and microwave packaging and cooking materials. Further, a need exists for the determination of the locus of origin of such volatile substance, any chemical change in the substance, the mode of production, transport and the final locus of such a substance that can migrate during cooking.